Nikola founder Trevor Milton has stepped down as executive chairman after a short seller accused the hydrogen and electric truck startup of misleading investors and overstating the value of a business deal.

Milton has also resigned from the company’s board, Nikola said in a statement on Sunday. The company has previously denied the allegations and threatened legal action against the research company that made them.

Stephen Girsky, a former vice chairman of General Motors and current board member at Nikola, will take over as chairman with immediate effect.

“The focus should be on the company and its world-changing mission, not me. I intend to defend myself against false accusations leveled against me by outside detractors,” Milton said in a statement posted on Twitter.

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Hindenburg Research, which makes money by betting against companies, published a report earlier this month alleging ​a range of misdeeds at Nikola, including that Milton presented ​a prototype truck as being closer to market than ​it actually was. The report also said that Nikola mischaracterized a business deal as being worth more than it was.

Shares in the company have dropped by more than 19% since the report was published by Hindenburg.

“These allegations by the short seller are false and misleading, and designed to manipulate the market to profit from a manufactured decline in Nikola’s stock price,” Nikola said earlier this month in response to a number of Hindenburg allegations.